Traditionally natural gas is compressed and sent down pipelines under high pressure to transport the gas to market. When the gas has arrived at its use point; the pressure of the natural gas is reduced in one or more control valves and/or pressure regulators to its final pressure for consumption. The available energy from the reduction in pressure of the natural gas is wasted in the control valves and pressure regulators as well as any small chilling effected caused by the flow of natural gas through these devices.
In the past, advantage has been taken of this wasted energy by facilities utilizing the energy and refrigeration effect of expanding the natural gas. One such facility was designed and constructed by Airco Industrial Gases' Cryoplants Division in the early 1970's in Reading, Pa. for UGI Corporation. It employed a natural gas pressure reduction station ("Letdown Station") to make liquefied natural gas ("LNG"). A majority of the natural gas entering the plant under high pressure from the transportation pipeline was cooled and sent to an expansion turbine where energy and refrigeration were generated. The remainder of the stream was subsequently cooled with the refrigeration and a portion liquefied. The liquefied portion was then passed to a storage tank as LNG product. The natural gas that was not liquefied was warmed, collected and sent to the low pressure main at a lower pressure than the high-pressure main.
Many industrial gas producers have taken advantage of LNG to reduce the power consumption in manufacturing merchant liquid products. British Oxygen Company built one such plant for Commonwealth Industrial Gases in Dandenong, Australia. In this facility, LNG at low pressure is vaporized employing warmer gaseous nitrogen at high pressure to liquefy the nitrogen gas. The power required to make the liquid nitrogen in this type of plant is greatly reduced from the power required by conventional nitrogen gas expansion merchant liquid plants that are used throughout the industrial gas producing industry employing nitrogen gas expansion. By combining a letdown station plant making LNG with a plant vaporizing LNG to make merchant gas liquids, a new type of Letdown Liquefier can be made to utilize the energy and refrigeration generating capabilities of natural gas letdown stations to make merchant liquid products.
In the mid 1980s, there was development work on a letdown station liquefaction plant to produce merchant liquid nitrogen, oxygen and argon. The design included removal of the H.sub.2 O and CO.sub.2 from the entire natural gas feed stream. In this concept, the majority of the natural gas was expanded with the rest liquefied. Subsequently, nitrogen was cooled by vaporizing and warming the partially liquefied natural gas. When throttled, the nitrogen became liquefied and was sent to the air separation unit to make merchant products. Integrating a letdown station liquefaction section directly with the air separation unit was also explored using air as the refrigeration sink, but this was dropped due to safely concerns with the air/natural gas processing proximity. No facility was constructed to practice this concept.
Among the objects and advantages of the present invention is to provide systems for producing liquid merchant gas such as nitrogen, oxygen and argon by employing the refrigeration capabilities of natural gas expansion and the energy recovered from letting down pressure through a letdown liquefaction process instead of a natural gas control valve or a pressure regulator.
It is among the further objects and advantages of the present invention to provide methods and systems for producing liquid merchant gases with less power consumption by recovering both refrigeration and energy from a natural gas stream.
An additional object and advantage of the present invention is to provide methods and systems for producing liquid merchant gases with additional liquid reflux generated by the inventive systems that increases the amount of product argon and oxygen.
A further object and advantage of the present invention is to provide methods and systems for producing liquid merchant gases with reduced capital expenditure resulting from recovering both refrigeration and energy from a natural gas stream.